1. Field of the Invention
The invention relates to a circuit and a method for adjusting a control loop to an input signal, in particular for a signal receiver.
2. Description of the Prior Art
One or a plurality of control loops are provided in many technical apparatuses, in particular in signal receivers.
FIG. 1 shows a control loop according to the prior art. A subtractor of the control loop subtracts a feedback signal xR, which is output by an internal loop filter, from an input signal xe present at an input E. The control difference or control deviation Xd generated by the subtractor passes via a controlled system to an output A of the control loop. The output signal xa of the controlled system, or the controlled variable, is filtered by the loop filter and output as feedback signal xR to the subtractor. The loop filter or the controller feeds the output signal of the control loop back to the input. The loop filter or the controller is chosen in a manner dependent on the controlled system to be controlled. The control loop according to the prior art as illustrated in
FIG. 1 contains a PID controller composed of a proportional (P) controller, an integral (I) controller, and a derivative (D) controller.
The feedback signal output by the PID controller or the loop filter is produced from the output signal as follows:
                                          x            R                    ⁡                      (            t            )                          =                                            K              P                        ·                                          x                a                            ⁡                              (                t                )                                              +                                    K              I                        ⁢                          ∫                                                                    x                    a                                    ⁡                                      (                    t                    )                                                  ⁢                                                                  ⁢                                  ⅆ                  t                                                              +                                    K              D                        ⁢                                          ⅆ                                                      x                    a                                    ⁡                                      (                    t                    )                                                                              ⅆ                t                                                                        (        1        )            where KP, KI, KD represent the transfer values or control parameters of the controller.
The transfer function of the loop filter reads as follows:
                                          H            R                    ⁡                      (            s            )                          =                                            K              P                        +                                          K                I                            s                        +                                          K                D                            ·              s                                =                                                                      K                  D                                ·                                  s                  2                                            +                                                K                  P                                ⁢                S                            +                              K                1                                      s                                              (        2        )            
FIG. 1 shows a control loop having a PID controller, in which a proportional controller, an integrator controller and a derivative controller are connected in parallel in a loop filter. A loop filter of this type can be extended by connecting in parallel further signal branches in which, by way of example, a plurality of integrators or differentiators are connected in series. The choice of the loop filter or controller depends on the controlled system to be controlled. Controlled systems having a delaying transfer response require an I or PI controller. Although controlled systems having an integration action managed with a P controller, certain additional improvements are possible with a PI controller. In many control tasks, a PID controller is advantageous with regard to realization complexity, handling and achievable effect.
The control loop according to the prior art as illustrated in FIG. 1 may be constructed in digital or analogue form. Control loops are used in particular in the signal receivers. These control loops have to be set differently depending on different requirements. In the case of adjustment to an input signal, the control loop is firstly set, in a so-called acquisition mode, in such a way that it is as fast as possible, that is to say in such a way that it minimizes the control deviation as rapidly as possible. Afterwards, the controller of the control loop is set in a so-called tracking mode in such a way that it is sluggish or slow, in order to effect good suppression of disturbances of the control deviation. The controller or the loop filter is set with the aid of the control parameters KP, KI, KD. The controller has a high frequency bandwidth in the acquisition mode and a small frequency bandwidth in the tracking mode. The control loop is fast in the acquisition mode, and the control loop suppresses noise disturbances well in the tracking mode.
In many applications, an integrated circuit contains a multiplicity of control loops. By way of example, it is customary to provide, on a digital receiver, a control loop for analogue gain setting, a further control loop for digital gain setting, a control loop for carrier frequency setting, a control loop for carrier phase setting and, finally, a further control loop for clock phase setting. Moreover, further control loops are provided for channel estimation within a receiver. The setting of the respective control parameters brings about a change in the limiting frequency fg of the respective control loop.
FIG. 9 shows the step response of a control loop according to the prior art. If the step signal illustrated in FIG. 9a is applied to a control loop according to the prior art, the signal profiles illustrated in FIG. 9 are produced. The curve I shows the step response of a conventional control loop operating in the acquisition mode, while the curve II illustrates the step response of a conventional control loop in the tracking mode. FIG. 9b shows the step response at the output of the loop filter. If the control loop in the tracking mode slowly approaches the desired value of 1, in the acquisition mode the signal fluctuates greatly around the desired value of 1. In conventional control loops, the control loop is firstly in the acquisition mode and is then changed over to the tracking mode. This is done by changing over between two sets of control parameters kp, ki, kd for setting the loop filter. The hard changeover between two sets of control parameters results in no minimization of the control deviation integrated in the time profile, that is to say of the energy of the control error or of the control difference signal xd.